An endoscope configured such that a light-distribution window through which illumination light for illuminating an observation target area is emitted and an observation window through which the illumination light reflected from the observation target area is received are provided at a tip portion of an insertion tube of the endoscope is known. The insertion tube of the endoscope of this type is formed in a shape of an elastic tube, and a light guide for introducing the illumination light to the tip portion is provided in the inside of the insertion tube. In the observation window, an objective lens is provided, and object light reflected from the observation target area is received by an image pickup device, such as a CCD (Charge Coupled Device), via the objective lens. The object light received by the image pickup device is subjected to signal processing and is displayed as a picked up image on a monitor. As a result, an operator of the endoscope becomes able to operate the endoscope while observing the image displayed on the monitor.
In general, as a monitor on which an image is displayed, a monitor having a laterally long aspect ratio, such as 4:3 or 16:9, is used. Therefore, if an aspect ratio of an emitted light distribution of illumination light emitted from the light guide does not coincide with the aspect ratio of the monitor, it becomes impossible to use the whole monitor screen effectively or the brightness of the image decreases because an area not displayed on the monitor is illuminated.
Japanese Patent Provisional Publication No, 2009-207529A (hereafter, referred to as patent document 1) describes an endoscope capable of obtaining a bright image. The endoscope described in patent document 1 includes a ring-shaped light guide in an insertion tube of the endoscope and is provided with a transparent cap at a tip portion of the insertion tube. The illumination light emitted from the light guide propagates through the inside of the cap, and is emitted from the tip portion. Around the periphery of the cap, an inclined surface is formed, and the inclined surface functions as a convex lens. Therefore, the illumination light which has propagated through the inside of the cap and passed through the inclined surface is emitted from the tip portion in a converged state. As a result, it becomes possible to prevent the illumination light from being scattered.
Japanese Patent Publication No. 4704386 (hereafter, referred to as patent document 2) describes an endoscope which includes a light guide having an exit end face of which cross sectional shape has different lengths between a left and right direction and a longitudinal direction (i.e., an elliptical shape). At a tip portion of an insertion tube of the endoscope of patent document 2, a transparent cap is provided. At a position of the transparent cap corresponding to the exit end face of the light guide, a light scattering part which is formed in a recessed shape to scatter transmitted illumination light is provided. The light scattering part has different lengths between a left and right direction and a longitudinal direction in conformity with the cross sectional shape of the exit end face of the light guide, and has the negative refractive power which varies depending on the length. Therefore, it is possible to change the degree of scattering of the illumination light between the left and right direction and the longitudinal direction, and thereby it becomes possible to bring the intensity distribution of the illumination light close to the aspect ratio of the monitor.